Examining surface displacement (e.g., vibration) of an object or surface with optical radiation has advantages in many settings, for example in high-temperature or vacuum conditions where physical contact with the object or surface could easily damage expensive equipment or disrupt the desired vacuum conditions. In another example, it is advantageous to perform contactless, nondestructive testing of structural members or mechanical components, for example for ultrasonic movement of the members or components, or to search for defects. Among various techniques proposed to exploit these advantages, techniques that employ the Fabry-Perot interferometer, the photo-emf effect or photorefractive crystals appear most promising, largely due to their ability to detect nanometer-scale vibrations of rough surfaces in the presence of speckle (speckle is created when the surface under investigation is optically rough, such that laser illumination of the surface and collection of backscatter results in speckle). Nonetheless, each of these techniques also requires “referencing,” which utilizes signal and reference beams from the same wavefronts during direct interferometric detection; for practical applications, this referencing requires additional optical components that often misalign while detecting intensity changes or speckle patterns in the optical radiation, particularly in the presence of mechanical vibration or movement. Misalignment may cause critical failures, because altering the angular relationship between signal and reference beams also changes the grating spacing on the detector. Prior art techniques (for example utilizing the photo-emf effect) are very sensitive to operating at precisely the right grating spacing.
Prior art devices that measure speckle patterns or changes in incidental optical radiation also utilize the power in the optical radiation to drive the output signal. Such devices are problematic because, for example, the power available to these devices is dependent on the detecting area; the electronic output depends on the detecting area so that scaling the device to smaller size results in lower output. Among other drawbacks, this impedes vibration detection since the detecting area must be smaller than the speckle size to avoid signal attenuation and to avoid averaging of variations across the sensing element.
Prior art optical detectors are also problematic whenever a small detecting area is needed and the intensity of the optical radiation is weak. For example, in such situations, photodiodes and photodiode arrays generate very small currents, from micro-amps to nano-amps, that are very hard to measure; they also generate signals that are significantly impacted by noise and interference. Improvements to optical detectors are therefore needed.